Randomised Controlled trials like you’re asking for are neither ethical nor practical in this situation. Even the Cass report stated that. Patients and doctors will know PDQ whether puberty is happening or not.

You’re right that more data is needed. More data is always needed, especially on anything regarding a marginalised group. And, in many of these situations where we know the outcome of puberty is irreversible, makes transitioning afterwards more difficult, with a decent threat of mental health decline without the treatment, waiting around and doing nothing is more harmful than pausing puberty temporarily, where, based on the 30 years worth of research done for puberty blockers to treat precocious puberty, we see the most likely risks are for them to wind up a little shorter than they might have, and maybe fatter.

If you’re worried the teenagers receiving this treatment may become sterile, the above linked precocious puberty article found no evidence, but here’s an article on a recent study where they used a placebo on rats (because, again, we’ll never have a randomised controlled trial done on humans). It adds to the body of data that shows reproductive activity returns to normal very quickly after stopping treatment, for the teens who do discover they’re OK with their assigned gender identity. We also shouldn’t ignore the good percentage of teens who realise they are trans, and benefit from this in more ways than just buying time.